Three approaches are commonly employed in implementing DC-to-DC converters—electronic circuits that converts a battery or DC voltage source to a different DC voltage. These methods comprise linear regulation, inductive switching regulators or so-called “switch-mode power supplies,” and switched capacitor converters, also known as charge pumps. Of these methods, the charge pump is valued for its simplicity, cost effectiveness, and relatively low noise operation. Under certain circumstances, the charge pump can operate at high conversion efficiencies, but not over the wide range of conditions that switched inductor based converters can achieve.
The operating principle of a charge pump is straight forward comprising a charging phase and a charge transfer phase which operate in alternating sequence. As shown in FIG. 1A, prior art charge pump doubler type circuit 1 comprises four MOSFETs, a flying capacitor not attached permanently to any specific supply voltage, and a grounded output filter capacitor. In the charging phase, battery-connected MOSFET 3 and grounded MOSFET 2 are turned on and allow conduct current and charge capacitor 5, electrically connecting the capacitor in parallel with the battery or voltage input to the circuit. MOSFETs 1 and 4 remain off during the charging phase of operation. This charging current is indicated in the schematic 1 by a dashed line and arrow. After some time, capacitor 5 charges to a voltage equal to the battery voltage Vbatt and the charging current subsides.
During the charge transfer phase, capacitor 5 is connected in series with the battery, specifically with its negative terminal shorted to the positive terminal of the battery achieved by turning on MOSFET 1. The voltage of the series combination of capacitor 5 stacked atop the battery input has a voltage of Vbatt+Vbatt=2Vbatt, or twice the battery voltage, hence the name “doubler” ascribed to this charge pump. This series circuit is simultaneously connected to output capacitor 6 by turning on MOSFET 4. Capacitor 5 then transfers its charge to output capacitor 6 until Vout→2Vbatt as shown by the solid line and arrows.
After the initial charging of output capacitor 6, the charge pump's operation becomes efficient since the only current flowing is that needed to replenish the charge lost on output capacitor 6 supplied to the load. As long as the desired output voltage is twice that battery voltage, i.e. 2Vbatt, the efficiency of doubler charge pump 1 is high, even up to 98%. Any deviation between the actual output voltage Vout and the charge pump's ideal output VCP=n·Vin will result in a loss of efficiency as given by the relation
  η  =                    V        out                    V        CP              =                  V        out                    n        ·                  V          in                    
The voltage differential between the charge pump lowers efficiency by causing one of the transistors to saturate a drop the incremental business. One common condition leading to lower efficiency in a doubler charge pump is “over-pumping” the output to a voltage higher than desired or required by the load.
Fractional Charge Pump Implementation: A common solution is to over-pumping is to employ a fractional charge pump, one that steps up by 1.5× rather than doubling its input. Such a fractional charge pump 20 as shown in FIG. 1B requires two flying capacitors 30 and 32, controlled by a matrix of MOSFET switches 21 through 27. Operation involves charging series-connected capacitors 30 and 31 through MOSFETs 21, 22 and 23 as illustrated by a solid line and arrow. After charging, the flying capacitors transfer charge from output capacitor 32 through conducting MOSFETs 24, 25, 26 and 27.
During charging, capacitors 30 and 31 are connected in series and charge to a voltage equal to Vbatt/2. During charge transfer, capacitors 30 and 31 are wired in parallel, connected in series with the battery input Vbatt with the series combination connected across output capacitor 32. The output voltage is charged to a voltage Vout→1.5Vbatt, a voltage 25% lower than the output of the doubler charge pump 1.
By employing a 1.5×-type fractional charge pump technique, efficiency is improved at lower output voltages but limited to a maximum of 1.5 times its input. Moreover, a 1.5× fractional charge pump, like the 2×-type charge pump, does not regulate voltage. As a result, its output voltage varies with its input which is undesirable in many applications.
Charge Pump Efficiency Considerations: Since a charge pump's output voltage varies with its input, it is not well adapted as a power converter and must often be combined with a linear regulator connected in series with the charge pump, to limit the output voltage swing. The linear regulator may be connected in either the input or output of the charge pump.
For example, a lithium ion input ranges from 4.2V to 3.0V during its discharge. Under such circumstances the output of a fractional 1.5× charge pump will vary in its output from 63V to 4.5V. A 2×-type charge pump doubler's output will vary from 8.4V to 6V under the same circumstances. If the load voltage is maintained at a fixed voltage, either by a linear regulator or because the load clamps the voltage across its terminals, then the efficiency will vary with the input voltage. The efficiency variation of linear regulated 1.5× and 2× charge pumps are summarized in the following table for a few commonly needed supply voltages. The output voltages of unregulated charge pumps are included in the table for reference along with a linear regulator with no charge pump, referred to in the table as a 1× converter.
ChargeUnregulated Voltage VCPLiIon Regulation Efficiency ηmax by VoutPumpMaxTypMin1.8 V2.5 V3 V3.3 V5 V2X8.4 V7.2 V6 V21%-30%30%-42%36%-50%39%-55%60%-83%1.5X6.3 V5.4 V4.5 V 29%-40%40%-45%48%-67%52%-73%80%-NA 1X4.2 V3.6 V3 V43%-60%60%-83%71%-NA 79%-NA NA
As shown, each output voltage exhibits a range of efficiencies that varies with the battery's voltage, starting with a lower efficiency when the Lilon cell is fully charged to 4.2V and improving as the battery discharges down to 3V. The term “NA” means not available, meaning that the charge pump is incapable of producing the desired output voltage over the full range of inputs. Efficiency has no meaning if the output falls out of regulation. It should be also be noted that the efficiency shown in the table, given by the relation:
  η  =                    V        out                    V        CP              =                  V        out                    n        ·                  V          in                    is the maximum theoretical efficiency of the charge pump, not taking into account losses in MOSFET resistance, switching losses, or other parasitic effects. The losses may further degrade efficiency by 3% to 6% below the theoretical maximum efficiency values shown.
From the table it is clear that efficiency is highest when the desired output voltage is close to the unregulated charge pump voltage, i.e. when Vout≈VCP. Lower output voltages therefore suffer from lower efficiencies because the charge pump is over-pumping the voltage to too high a value. For example a 1.8V volt output for a charge pump doubler has a peak theoretical efficiency of 30% while a 3V output has a conversion efficiency of 50%. Under the same circumstances, the fractional charge pump has a higher efficiency, 40% for a 1.8V output and 67% for a 3V output, because it is not pumping its output to as high a voltage as the doubler.
On the other hand, a fractional charge pump cannot output all the voltages commonly desired in a system. For example, a 1.5× charge pump cannot produce a 5V output over the full lithium ion range. At slightly above 3.3V the output voltage will sag below the desired 5V and the system may fail, meaning a 1.5× charge pump cannot be used reliably to produce a 5V regulated supply, despite having a higher efficiency when it is able to do so.
So if higher charge-pump multiples are used, e.g. n=2, the converter regulates over a wider voltage range but operates at lower efficiencies. If lower conversion factors of n are used, e.g. n=1.5 or even n=1, then the converter cannot supply the voltage over the full battery operating range unless the condition VCP(min)>Vout can be maintained.
One solution to the range versus efficiency tradeoff is to employ mode switching, i.e. to combine the doubler and fractional charge pumps into a single circuit, operating in 1.5× mode until the battery discharges and switching into 2× mode when the battery discharges. In this manner a higher average efficiency may be maintained over the battery voltage range. Such mode switching charge pumps capable of operating at two different values of “n”, in this case at 1.5× and 2×, are referred to as dual-mode charge pumps.
For outputs such as 3V and 3.3V even the 1× mode, or linear regulator only mode, may be used for some portion of time before the charge pump needs to turn on. By combining 1.5× and 1× mode charge pumps into a single charge pump, the resulting dual-mode charge pump is better adapted to lower voltage outputs than combining 2× and 1.5× modes.
Even more versatile, but slightly more complex a tri-mode charge pump, may operate in any of three modes, for example operating in step-down-only 1×-mode when the battery is charged, switching to 1.5× mode as the battery becomes discharged, and jumping into 2× mode if a higher voltage or current is temporarily demanded by the load. As one example, a tri-mode charge pump can drive 3.6V white LEDs as the back light in a cell phone using its 1.5× and 1× modes, and then momentarily switch into 2× mode whenever the 4.5V camera flash LEDs are needed.
An example of a tri-mode charge pump 35 is illustrated in FIG. 1C where the charging and discharging of flying capacitors 45 and 46 are controlled by a matrix of MOSFET switches. This matrix combines topological elements of charge pump doubler circuit 1 with fractional charge pump 20, along with the means by which the entire charge pump circuit may be bypassed to achieve IX pass-through operation.
Except in 1× bypass mode where the charge pump is not switching, tri-mode charge pump 35 operates by the same principal as single-mode charge pumps 1 and 20, i.e. by successively charging flying capacitors 45 and 46 to a voltage Vfly, then transferring their charge to output filter capacitor 49 as needed. In the 1.5× mode, the capacitors are series connected and each charged to a voltage of Vbatt/2 through conducting MOSFETs 36, 37 and 38 while all other MOSFETs remain off. In 2×-mode, each flying capacitor is placed in parallel with the battery and charged to a voltage Vbatt through conducting switches 36, 39, 42 and 38 while all other MOSFETs, including MOSFET 37 remain off.
The charge transfer mode is the same regardless whether flying capacitors 45 and 46 are charged to a voltage Vbatt or Vbatt/2. Conducting MOSFETs 40 and 42 connect the negative terminals of charged capacitors 45 and 46 to the input voltage Vbatt. Conducting MOSFETs 43 and 44 along with forward biased diodes 47 and 48 connect the positive terminals of charged capacitors 45 and 46 to the converter's output and to filter capacitor 49. Charge transfer there occurs so that Vout→(Vbatt+Vfly). If Vfly is charged to a voltage Vbatt, then Vout→2Vbatt and charge pump circuit 35 operates as a doubler. If Vfly is charged to a voltage Vbatt/2, then Vout→1.5Vbatt and circuit 35 operates as a 1.5×-type fractional charge pump.
To operate in 1× bypass mode, conducting MOSFETs 36, 42, 43, 44 and optionally 40 and 37 connect Vout directly to Vbatt. No switching action is needed in this operating mode.
So aside from the disadvantage of containing a large number of MOSFETs to implement the switching matrix, tri-mode charge pump 35 can adjust its mode to reduce over-pumping and improve operating efficiency at any given output voltage.
Limitations of Charge Pumps: Many systems today require more than one regulated output voltage. One solution to this problem is to step up the battery voltage with a charge pump and then regulate down to lower voltages using more than linear regulator as illustrated in schematic 50 of FIG. 2.
As shown charge pump 51 powered by Lilon battery 58 generates a voltage VCP which is stored on reservoir capacitor 57 and then regulated by linear regulators 51, 52, and 53 to produce various required regulated voltages Vout1, Vout2, and Vout3. Capacitors 54, 55, and 56 provide added filtering and improve regulator stability.
For example using a doubler for charge pump 51, linear regulators 51, 52 and 53 may be used to produce any desired voltage from 1V to nearly 6V. Using a fractional charge pump to implement converter 51, the guaranteed voltage VCP is limited to below 3V since a 1.5×-mode cannot reliably produce a 3V output and since some voltage, typically 300 mV, is lost as a voltage drop across the linear regulator.
Furthermore, if both positive, i.e. above ground, and negative, i.e. below ground supply voltages are required by the system, the approach of FIG. 2 cannot be employed and multiple charge pumps are required.
In summary, the limitation of today's charge pumps is that they produce a single-voltage single-polarity output. While the charge pumps output voltage may be varied in time by mode switching, it must always deliver a voltage VCP higher than the highest voltage required by the system. Such restrictions greatly limit the use of charge pumps, forcing designers to employ one charge-pump per load, undesirably increasing costs, component count, and printed circuit board space.
What is really needed is a multiple output charge pump voltage converter or regulator capable of producing any number of positive and negative supply voltages simultaneously with the minimum number of components.